The present invention relates to an optical fiber transmission system using soliton signals with wavelength division multiplexing in which the various wavelengths of the multiplex are selected so as to ensure that over a given interval the relative slip between the various channels is substantially equal to a multiple of the bit time.
The invention also relates to a method of transmission in such a system.
The transmission of soliton pulses or xe2x80x9csolitonsxe2x80x9d in the portion of an optical fiber that has abnormal dispersion is a known phenomenon. Solitons are pulse signals having a sech2 waveform. With pulses of this waveform, the non-linearity in the corresponding portion of the fiber compensates dispersion of the optical signal. Soliton transmission is modelled in known manner by the non-linear Schrxc3x6dinger equation.
Various effects limit the transmission of such pulses, such as the jitter induced by solitons interacting with the noise present in the transmission system, as described for example in the article by J. P. Gordon and H. A. Haus, in Optical Letters, Vol. 11, No. 10, pp. 665-667. This effect which is known as the xe2x80x9cGordon-Haus effectxe2x80x9d or as xe2x80x9cGordon-Haus jitterxe2x80x9d puts a theoretical limit on the quality or on the bit rate of transmission by solitons.
In order to be able to exceed that limit, it is possible to use synchronous modulation of soliton signals with the help of semiconductor modulators. That technique intrinsically limits the bit rate of the soliton link because of the upper limit on the passband of semiconductor modulators. Proposals have therefore been made for sliding guiding filter systems that make it possible to control the jitter of transmitted solitons, see for example EP-A-0 576 208. For the purposes of regenerating the signal on the line, proposals have also been made to use the Kerr effect in synchronous amplitude or phase modulators. Finally, proposals have been made to use saturable absorbers to regenerate soliton signals.
For the purpose of increasing the bit rate of optical fiber transmission systems using soliton signals, proposals have also been made to use wavelength division multiplexing (WDM). Under such circumstances, it is considered advantageous to use sliding guiding filters of the Fabry-Perot type which are fully compatible with wavelength division multiplexed signals. In contrast, the use of synchronous modulators or of saturateable absorbers for regenerating wavelength division multiplexed soliton signals is problematic because of the differences in the group velocities of the signals in the various channels.
An article by E. Desurvire, O. Leclerc, and O. Audouin, published Optics Letters, Vol. 21, No. 14, pp. 1026-1028, describes a scheme for allocating wavelengths which is compatible with using synchronous modulators. That article proposes allocating wavelengths to the different channels of the multiplex in such a manner that for given intervals ZR between repeaters, the signals in the various channels, or more exactly the bit times of the various channels of the multiplex, are substantially synchronized on arrival at the repeaters. This makes in-line synchronous modulation of all of the channels possible at given intervals with the help of discrete synchronous modulators. That technique for allocation the wavelengths of the multiplex is also described in French patent application No. 96/00732 of Jan. 23, 1996 in the name of Alcatel Submarine Networks. In the article, it is proposed to select a subgroup of channels that are synchronous not only at intervals ZR, but also at intervals that are submultiples of ZR.
Another article by O. Leclerc, E. Desurvire, and O. Audouin, published in Optical Fiber Technology, 3, pp. 97-116 (1997) specifies that the above wavelength allocation scheme can give rise to intervals ZR between synchronous modulators that are too great, or to spacing between channels in the multiplex that are too great. To mitigate that problem, the article observes that in such a wavelength allocation scheme, the bit times of subsets of the channels to be multiplexed are synchronized at intervals that are submultiples of ZR. Consequently, the article proposes regenerating subsets of the channels in the multiplex at shorter intervals.
Nevertheless, that solution requires the channels of the subset that is to be regenerated to be filtered out, and it causes the transmission system to lose its unique periodicity for all of the channels.
The present invention proposes a solution to the problem of synchronous modulation of wavelength division multiplexed soliton signals that is original and simple. It makes it possible to avoid the drawbacks mentioned above. The invention enables all of the channels in the multiplex to be modulated simultaneously.
More precisely, the invention provides an optical fiber transmission system using soliton signals with wavelength division multiplexing having a clock period T, in which the wavelengths, xcex1 to xcexn, of the various channels of the multiplex are selected in such a manner that at least at one point in the transmission system, the difference between the bit times of any two channels of the multiplex is substantially equal to a fraction kT/N of the clock period, where k is an integer, the system including, at least at said point, a synchronous modulator for modulating the soliton signals at a frequency N/T which is a multiple of the soliton clock frequency 1/T.
Advantageously, the wavelengths xcex1 to xcexn, of the various channels of the multiplex are selected so that at a plurality of points of the transmission system that are spaced apart by an interval ZR, the difference between the bit times of any two channels of the multiplex is substantially equal to a fraction kT/N of the clock period, the system including, at each of said points, a synchronous modulator for modulating the soliton signals at a frequency N/T which is a multiple of the soliton clock frequency 1/T.
In an embodiment, the wavelengths, xcex1 to xcexn, of the various channels of the multiplex are selected in such a manner that at least at one other point of the transmission system, the difference between the bit times of any two channels of the multiplex is substantially equal to the clock period T, the system including, at said at least one other point, a synchronous modulator for modulating the soliton signals at the soliton clock frequency 1/T.
Preferably, the wavelengths, xcex1 to xcexn, of the various channels of the multiplex are selected in such a manner that at a plurality of other points of the transmission system spaced apart by an interval ZR, the difference between the bit times of any two channels of the multiplex is substantially equal to the clock period T, the system including at each of said other points a synchronous modulator for modulating the soliton signals at the soliton clock frequency 1/T.
In an embodiment, at said point or at each of said points, the difference di between the bit times of a channel of wavelength xcexi of the multiplex and of the first channel of wavelength xcexl satisfies the relationship:
|dixe2x88x92kixc2x7T/N| less than T/4
where ki is an integer depending on the channel.
Advantageously, at said other point or at each of said other points, the difference di between the bit times of a channel of wavelength xcexi of the multiplex and of the first channel of wavelength xcex1 satisfies the relationship:
|di| less than T/4
Provision can also be made to ensure that for each wavelength xcexi of the multiplex, and for an interval ZR, the difference xcex94xcfx84i between the slip per unit length on channel i and on the first channel to satisfies the following relationship:
(kIxe2x88x92T/4) less than xcex94xcfx84ixc2x7ZR less than (kixc2x7T+T/4)
where ki is an integer depending on the channel.
When the system is a system without dispersion slope compensation, the wavelengths, xcex1 to xcexn, of the various channels of the multiplex are advantageously selected so that for an interval ZR:
(kixc2x7Txe2x88x92T/4) less than Dxe2x80x20xc2x7xcex94xcexi1xc2x7(xcex94xcexi1+2xc2x7xcex94xcex10)xc2x7ZR/2 less than (kixc2x7T+T/4)
where:
ki is an integer depending on the channel;
Dxe2x80x20 is the dispersion slope;
xcex4xcexi1 is the wavelength difference between channels i and 1; and
xcex94xcex10 is the wavelength difference between the first channel and the wavelength xcex0 having zero dispersion.
If the system is a system having dispersion slope compensation, then the wavelengths, xcex1 to xcexn, of the various channels of the multiplex are preferably selected so that for an interval, ZR:
(kixc2x7Txe2x88x92T/4) less than Dmxc2x7xcex94xcexi1xc2x7ZR/2 less than (kixc2x7T+T/4)
where:
ki is an integer depending on the channel;
Dm is the mean dispersion slope for wavelengths xcex1 to xcexn; and
xcex94xcexi1 is the wavelength difference between channels i and 1.
In an embodiment, said modulator at the frequency N/T is an intensity modulator of modulation depth that is selected so as to compensate the effects of modulation at the frequency N/T on modulation depth compared with a modulator operating at the frequency 1/T.
In another embodiment, said modulator at the frequency N/T is an intensity modulator having modulation depth selected so as to compensate for the effects of modulation at the frequency N/T on modulation depth compared with a modulator operating at the frequency 1/T, which effects are expressed by the following formula:
IMN=20xc2x7log(N)xe2x88x9210xc2x7log[N2xe2x88x921+10IM1/10]
where:
IMN is the modulation depth of the modulator at the frequency N/T; and
IM1 is the modulation depth of the modulator at the frequency 1/T.
The invention also provides a method of transmitting soliton signals in an optical fiber system with wavelength division multiplexing, in which the bit times of the various channels xcex1 to xcexn of the multiplex are selected in such a manner that at least one point of the transmission system, the difference between the bit times of any two channels of the multiplex is substantially a submultiple T/N of the substantially synchronous clock period at least one point, the method including at least one step of synchronously regenerating the signals of the channels of the multiplex at said point by synchronous modulation at a frequency N/T which is a multiple of the soliton clock frequency 1/T.
Preferably, the wavelengths, xcex1 to xcexn, of the various channels of the multiplex are selected in such a manner that at a plurality of points of the transmission system that are spaced apart by an interval ZR, the difference between the bit times of any two channels of the multiplex is substantially equal to a submultiple T/N of the clock period, the method comprising a plurality of steps of synchronously regenerating the signals of the channels in the multiplex at each of said points by synchronous modulation at a frequency N/T which is a multiple of the soliton clock frequency 1/T.
Advantageously, the wavelengths, xcex1 to xcexn, of the various channels of the multiplex are selected in such a manner that at least one other point of the transmission system, the difference between the bit times of any two channels of the multiplex is substantially equal to the clock period T, the method comprising a step of synchronously regenerating the signals of the channels of the multiplex at said at least one other point by synchronous modulation at the soliton clock frequency 1/T.
It is possible to select the wavelengths xcex1 to xcexn of the various channels of the multiplex so that the difference between the bit times of any two channels of the multiplex is substantially equal to the clock period T at a plurality of other points of the transmission system remote from any interval ZR, the method comprising a plurality of steps of synchronously regenerating the signals of channels in the multiplex at each of said other points by synchronous modulation at the soliton clock frequency 1/T.
It is also possible to select the wavelengths xcex1 to xcexn of the various channels of the multiplex in such a manner that at said point or at each of said points, the difference di between the bit time of a channel of wavelength xcexi of the multiplex and of the first channel of wavelength xcex1 satisfies the relationship:
|dixe2x88x92kixc2x7T/N| less than T/4
where ki is an integer depending on the channel.
Preferably, the wavelengths, X1 to xcexn, of the various channels of the multiplex are selected in such a manner that at said other point or at each of said other points, the difference di between the bit times of a channel of wavelength xcexi of the multiplex and of the first channel of wavelength xcex1 satisfies the relationship:
xe2x80x83|di| less than T/4
Advantageously, the wavelengths, xcex1 to xcexn, of the various channels of the multiplex are selected in such a manner that for all of the wavelengths xcexi of the multiplex, and for an interval ZR, the difference xcex94xcfx84i between the slip per unit length on channel i and on the first channel satisfies the relationship:
(kixc2x7Txe2x88x92T/4) less than xcex94xcfx84ixc2x7ZR less than (kixc2x7T+T/4)
where ki is an integer depending on the channel.
If the system is a system without dispersion slope compensation, the wavelengths X1 to xcexn of the various channels of the multiplex are preferably selected so that for an interval ZR:
(kixc2x7Txe2x88x92T/4) less than Dxe2x80x20xc2x7xcex94xcexi1xc2x7(xcex94xcexi1+2xc2x7xcex94xcex10)xc2x7ZR/2 less than (kixc2x7T+T/4)
where:
ki is an integer depending on the channel;
Dxe2x80x20 is the dispersion slope;
xcex94xcexi1 is the wavelength difference between channels i and 1; and
xcex94xcex10 is the wavelength difference between the first channel and the wavelength xcex0 at which dispersion is zero.
If the system is a system with dispersion slope compensation, the wavelengths xcex1 to xcexn of the various channels of the multiplex are advantageously selected so that for an interval ZR:
(kixc2x7Txe2x88x92T/4) less than Dmxc2x7xcex94xcexi1xc2x7ZR less than (kixc2x7T+T/4)
where:
ki is an integer depending on the channel;
Dm is the mean dispersion slope for the wavelengths xcexi to xcexn; and
xcex94xcexi1 is the wavelength difference between channels i and 1.
In an embodiment, the synchronous modulation at the frequency N/T is intensity modulation with modulation depth selected to compensate the effects of modulation at the frequency N/T on modulation depth compared with a modulator operating at the frequency 1/T, said effects being expressed by the following formula:
IMN=20xc2x7log(N)xe2x88x9210xc2x7log[N2xe2x88x921+10IM1/10]
where:
IMN is the modulation depth of the modulator at the frequency N/T; and
IM1 is the modulation depth of the modulator at the frequency 1/T.